1. Technical Field of the Invention
The present invention relates to an extrusion die such as a solid die or the so-called "port-hole" die, which are adapted for use in extruding small-, medium- or large-sized articles such as the multi-bored flat tubes in a heat exchanger made of aluminum or its alloy, and also the present invention relates to a method of extruding such articles by using the extrusion die.
2. Prior Art
FIG. 17 shows an example of aluminum tubes which constitute a heat exchanger employed for instance in the air conditioning system. Among the various methods of manufacturing such a tube 1, the extrusion method is advantageous in that a high pressure resistance can be enhanced to the tube.
The die assembly, for example the "port-hole" die, used to extrude the tubes does comprise in general a male die 51 mating a female die 52 as shown in FIG. 19a. The male die forms a hollow space extending through and longitudinally of the tube 1, whereas the female die forms a periphery of said tube.
FIGS. 19a and 20 illustrate the prior art female die 52 which is an integral piece made of a die steel. This female die has a recess 53, a central bearing hole 54 and a bell-shaped portion 55 which are arranged close to and in axial alignment with one another. The recess 53 provides a fusion chamber, and the bearing hole 54 directly contributes to the control of extrusion process.
A circumferential edge defining the bearing hole 54 will be abraded and worn out as the extrusion process is repeated. The one-piece female die 52 has to be replaced as whole with a new one, thereby undesirably raising the running cost of the extrusion process, especially where the medium- or large-sized die is used.
The tubes 1 for the heat exchanger are so small and so precise that the bearing hole tends to be deformed due to a high pressure of the extruded material. Therefore, it has been difficult to continuously ensure a high precision in shape and dimension of the flat tubes 1 such as shown in FIG. 17, in which their width "B" and height "H" are 10-20 mm and 3-7 mm, respectively. Similar problem of the impaired accuracy in shape and dimension is also found when extruding the medium- or large-sized articles.
has been proposed to make the female die 52 from a hard material such as a ceramics or a hard metal (that is "cemented carbide"). The female die made of such a material might be more resistant to abrasion and the frequency of changing the female die would be reduced. Further, the deformation of the die during the extrusion process would also be diminished to manufacture the tubes 1 of a higher precision in their dimension and shape.
Since the ceramics and the hard metal are however too expensive, it has not been feasible to supply the female die 52 at a reasonable price. In case of the medium-or large-sized die, its material cost would be raised to an almost intolerable extent.